Cable systems, e.g. cable systems used for Cable TV (CATV), are one prevalent and effective way of communicating information. A cable system can be broadly divided into three parts: source generation; combination; and distribution. Source generation consists of creating streams of audio, video, and computer data from different types of media. For example, the data streams (or data) can be read from a video tape player (VTR), laser disc, or computer hard drive. Data may also be transmitted to the cable system from cameras in a television studio, satellites, fiber optics, the cable system itself, or through the air and received via antennas.
In the combination part, the cable system uses a series of modulators and combiners to bring the streams of audio, video, and computer data, onto a common carrier: the coaxial cable. A modulator takes a data signal (e.g., audio, video, and/or computer/digital) and modulates it into a 6 MHz frequency slot. The combiners combine the output of the modulators together. By carefully allocating the frequency spectrum, the interference of each modulated signal can be minimized. The modulation and combination matrix within these systems can become complex. Each building which contains a combination system is called a "head-end".
The combination of frequency slots, and the data within the slots, defines the channel lineup for the cable system. A channel lineup is a list of the channels, e.g. the signal frequencies, and the program, e.g. the TV shows, that the cable company offers to its customers. A single cable system will often serve several different areas within a town or county and provide each area with slightly different channel lineups. For example, a Spanish language channel lineup might be broadcast to one geographic area with a large Spanish speaking population, while other programs (i.e., in English) would be transmitted to other locations. As another example, meetings in town halls would be transmitted only to the customers within the respective town in which the town hall is located.
Once the audio, video, and computer data channels have been modulated and combined together, the resulting frequency spectrum is distributed to the cable systems subscribers other the distribution part of the cable system. Distribution is done through a tree and branch system of fiber optics and coaxial cable. The cable system will translate the cable frequencies into optical frequencies and transmit the data over fiber optic cables through a series of fiber optic filters, spitters, and repeaters. The use of fiber optics allows the cable system to transmit its signal over long distances with little loss or outside interference. Wherever the fiber passes pockets of subscribers homes, translators are put in place to translate the fiber frequencies back to the cable spectrum. The data then travels over coaxial cable through cable amplifiers and splitters into each subscribers home (drop). Amplifiers in the distribution system have bidirectional capability. Just as signals are modulated at the cable system head-end and broadcast to the homes, signals may also be modulated onto assigned frequency slots at a customer drop and transmitted back to head-ends.
Cable companies are trying to connect one location to another with a cable link without running a dedicated link. In order to do this, connections are manual connected and disconnected at the head end.
Cable systems have several ways to connect data sources to modulators. The majority of video channels in a cable system will typically be received from a satellite and be connected directly into modulators. A satellite dish will receive all of the data channels being broadcast on a particular satellite. These data channels will be sent to satellite receivers and descramblers which convert the satellite data to baseband audio and video. The resulting baseband audio and video will then be sent to dedicated modulators.
Cable systems also receive and distribute locally originated video. This video typically comes from video tapes or is received live from a television studio. The video can consist of news broadcasts, local commercials, or public events such as town hall meetings or sports games. This video does not generate as much revenue for the cable system and several video sources will often share the same modulator. The video sources can be either connected to the modulators manually by plugging and unplugging wires; through a cross-point switch; or through other relays. The cross-point switches and relays may be controlled manually by push-buttons, through timers, or through RS-232 connections to computers and computer terminals.